1. Field of the Invention
Exemplary aspects of the present invention generally relate to an electrophotographic image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photoconductive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
In known image forming apparatuses, a transfer method known as a direct current (DC) transfer method, in which a direct current bias is applied to a transfer device, is widely employed to transfer a toner image onto a recording medium.
In recent years, there is also known an alternating current (AC) transfer method in which a superimposed bias (also known as an AC bias) is applied to the transfer device. In the AC transfer method, the superimposed bias is composed of an alternating current (AC) voltage superimposed on a DC voltage. It is to be noted that thereafter, the transfer method in which the superimposed bias is used as a transfer bias is referred to as an AC transfer. The AC transfer method is more advantageous than the DC transfer method for a recording medium having a coarse surface. It is known that the AC transfer method can enhance transferability and prevent a disturbance of toner image such as dropouts.
Although advantageous and generally effective for its intended purpose, in the AC transfer method, toner may not be transferred well if the same transfer bias used in a monochrome mode for forming a monochrome image is applied in a color mode for forming a multicolor or full-color image.
In the known DC transfer method, the level of DC voltage supplied as a transfer bias is changed when forming a color image (in the color mode), such as JP-2004-177920-A. In this approach, in order to prevent improper transfer of toner derived from a difference in the amount of toner in the toner image, a transfer voltage for forming a color image is configured greater than a transfer voltage for forming a monochrome image.
However, the transferability does not increase proportional to the transfer voltage in the AC transfer method. More specifically, simply increasing the transfer voltage does not transfer toner well onto a recording medium for a color image that contains a large amount of toner.
In view of the above, there is thus an unsolved need for an image forming apparatus capable of maintaining good transferability regardless of color imaging or monochrome imaging.